For the formation of inorganic salts a variety of conventional precipitation techniques are known which are mostly always batch-wise processes. This can be easily demonstrated with procedures which are used for the preparation of silver halide salts dispersed in a hydrophilic protective colloid- The most common approach of the batch preparation technique is the single-jet (or single-run) procedure where a silver salt solution is added in a controlled or uncontrolled way into a reaction vessel containing a halide salt. Hereby silver halide nuclei are formed in a first reaction step which is followed by a growth process for the formation of the ultimate silver halide grains. An alternative and more widely used batch process for the formation of silver halide crystals is the double jet precipitation technique whereby a silver salt and a halide salt solution are concurrently added to a reaction vessel containing a dispersing protective colloid. In this process too one can distingish a nucleation and a growth stage which is always undertaken under conditions of rapid stirring of reactants while the volume in the reaction vessel is continuously increasing during the preparation procedure. The double-jet method offers silver halide crystals with narrower grain size distribution and allows free alteration of the halide composition of crystals according to their growth. Double-jet precipitation techniques are described, for example, in U.S. Pat. No. 3,801,326, U.S. Pat. No. 4,046,376, U.S. Pat. No. 4,147,551, and U.S. Pat. No. 4,171,224.
These techniques can be used for the production of many different kinds of crystals having various morphological modifications which can be cubic, tabular, etc in a regular or irregular way. A crystal structure which is increasingly used in photographic industry is the tabular form mainly on account of the increased covering power and further for the increasing surface area of these crystals in respect to their volume. The later aspect has its positive influence on the (spectral) sensitivity which is like the enhanced covering power a very important argument for their use. And next to the existing and ever growing need for more uniform non-tabular crystals in order to get a higher sensitivity and a better granularity and sharpness, the same trend exists for the tabular crystal structure. In the last decade various proposals were filed in many patents which can be divided in different groups on the basis of the used method.
One of the methods starts with the formation of small instable nuclei which are formed in a nucleation chamber outside the precipitation vessel as, for instance, described in JN-A 02.172.815, JN-A 02.172.816 and JN-A 02.172.817. Increased uniformity of crystals can be realized if the nuclei are prepared in a low molecular gelatin (MW&lt;25000) (EP-A 0 610 597) or if the soluble silver and halide salts are mixed together in the nucleation room via a membrane as proposed by Saito (JN-A 05.061.134 and JN-A 06.086.923). Another approach for the preparation of monodisperse tabular crystals uses a modificator added directly after the nucleation phase and adsorbing specifically on (for instance) certain (100- or 111-) lattice sites as described in JN-A 09.218.476 or by Jablonka et al in Chemik.(1998)51(2)44-46. Another method for the preparation of monodisperse tabular crystals was recently described by Larichev et al in Zh.Nauchn. Prikl.Fotogr.,(1996)41(4)1-4. These authors stated that the physical ripening of a fine grain emulsion in the presence of KCNS and at low pBr (of 2-3) gives very uniform tabular crystals by coalescence. The most common method to get a monodisperse tabular crystal population is realized by controlling the pBr during the growth and if desired, the temperature during the physical ripening and growing phase as for instance described in WO92/07295.
But searching for methods to get increased monodispersity raises also the question for a more uniform thickness of the tabular crystals. However this crystal characteristic can be controlled (as explained by Moskinov et al at the IS&T 49.sup.th Ann.Conf. (1996)126-128) by the gelatine concentration and the temperature during the nucleation, the pBr during twinning and growth, etc. But particularly if there is a need for thinner tabular crystals the monodispersity will become a problem. Other methods like decreasing the gelatine concentration or using gelatin with lower molecular weight lead to thinner tabular crystals but with loss of uniformity. Further an increased turbulence during precipitation will result in thinner and more uniform crystals but the effect is small and this parameter is a negative factor in scaling-up procedures. It is also possible to interact in the nucleation phase or during the first ripening step in order to get a very thin nucleus as is proposed by Maskasky who uses an amino-pyrimidine grain growth modifier at low pBr and at room temperature in order to get ultra-thin tabular grains (U.S. Pat. No. 5,411,851). But this method is more focused on `thin` than on `monodisperse and thin` tabular.
The procedure leading to the needed monodisperse and thin tabular crystal population without the side effects mentioned before is using a (very) low concentration of the reactants during growth which results in an anisotropic growth. This is explained by a mechanism of anisotropic growth which is given by Sugimoto in Phot.Sci.Eng.,28(1984) 127 and J.Imag.Sci.,33(1989)(6) 203-5 and afterwards further described by Hosoya and Tani (in T.Tani, `Photographic Sensitivity`, Oxford Series nr 8, Oxford University Press, 1995, pag. 39-41). It can be understood that thin tabular crystals formed in this way can be kept homogene if the growth takes place at very low concentration of the silver and halide salt reactants which means that the preparation procedure itself is time-consuming and very inefficient while the emulsion has to be concentrated afterwards. This extra working-step can be carried out with, for instance, ultrafiltration or dialysis and gives the emulsion with the requested characteristics. Combining this concentration step together with the precipitation as described in JN-A 02.172.816 and JN-A 02.172.817 does not give the wanted results and leads to a tabular emulsion with thicker emulsion crystals.